The present invention relates generally to a linear electromagnetic machine and particularly, although not exclusively, to apparatus for producing linear motion.
Although such machines are frequently referred to as xe2x80x9clinear actuatorsxe2x80x9d the present invention also comprehends a machine which, although capable of motion, is adapted to maintain a fixed position against a varying force.
A number of designs of linear electromagnetic actuator, (sometimes called a linear motor), are known. Various configurations of previously known machines are described in WO93/01646, which relates to a linear electromagnetic device having axial symmetry and formed as a piston-in-cylinder machine. The principal advantage of that form of construction is that the strong attractive forces between the magnetic elements of the stator and the magnetic elements of the armature are balanced about a central axis, so that the bearings of the machine do not need to withstand resultant magnetically created forces, as is the case with a unilateral so-called linear motor.
A further advantage of an axially-symmetric construction is that the magnetic fields within the machine are contained within an outer casing (which may be steel or other suitable ferromagnetic material) and they can be so arranged as to intersect electrical coils of the machine with a high degree of efficiency. Moreover, in an axially-symmetric construction of cylindrical form it is convenient and practical for the moving member, or armature, to carry a sliding seal between it and the stationary member, or stator of the actuator so as to be able to contain a volume of fluid, at least to one side of the armature, which can act as a supplementary fluid actuator or damper.
For many purposes the most appropriate form of such a machine is that using a permanent-magnet armature and a surrounding stator carrying coils through which electrical currents pass. Preferably the electrical currents are controlled in magnitude and sign by an electronic control system. This configuration allows the heat generated in the windings to pass easily to the external environment and it simplifies the electrical connections to the control system. Important design parameters of such machines are the conformation of the magnetic fields of the armature and the stator, and the choice of the most suitable diameter of the machine for a given stroke and thrust rating.
The thrust generated by an electromagnetic machine may be described by:
T=B*I*L 
where T is the thrust in Newtons, B is the flux density in Tesla, I is the current in the wire in Amps and L is the length of the wire in Meters. It follows directly from this relationship that the volume of copper that must be intersected by the magnetic flux is given by
V=T/(B*d) 
where V is the volume in cubic meters and d is the current density in the wire in amps per square meter. It also follows that the electrical power required to produce this thrust when the machine is stationary is given by
W=T*r*d/B 
where W is the power in Watts and r is the resistivity in ohm meters.
It will be appreciated, therefore, that the performance of an actuator can be improved by ensuring that the magnetic flux is concentrated as much as possible in the regions where it intersects the coils.
According to one aspect of the present invention, therefore, there is provided a linear electromagnetic machine for generating a force acting between two members relatively movable along an axis, one of which is capable of producing a magnetic field the direction of at least part of which extends substantially radially of the said axis and the other of which comprises or includes a conductor in which an electric current can flow substantially perpendicularly with respect to the said magnetic field over at least part of its path whereby to generate a force between the said two members substantially parallel to the said axis, characterised in that the said means for generating the radial magnetic field comprises a permanent magnet orientated with its direction of magnetisation substantially parallel to the said axis associated with pole pieces for its north and south poles from circumferential surfaces of which the generally radial magnetic field extends.
Early forms of linear electromagnetic machine were of the xe2x80x9cironlessxe2x80x9d type. That is to say, the copper windings were located directly in the air gap of the magnetic circuit. The air gap had therefore to be large in order to contain, in an acceptable length, the minimum copper volume necessary to produce the required thrust. In order to force the flux across such a large air gap the magnetic length of each associated permanent magnet was also large, so that the volume, weight and cost of the permanent magnetic material was significant. In embodiments of the present invention it is preferred that the axial length of the polepieces should be kept as short as possible in order to maximise the flux density. However, for reasons which will become clearer below, the axial length of the magnets should preferably be less than, and in any event, not substantially greater than the axial length of the polepieces. In attempting to minimise the polepiece axial length, however, a constraint is imposed by the condition that the radial thickness of the air gap (including the copper in an ironless machine) must be approximately half the length of the magnets in order for these to operate at maximum efficiency. There is a limit to the radial xe2x80x9cthicknessxe2x80x9d to which the coils can be reduced and this has to be balanced against the need to minimise the axial length of the polepieces. A particularly effective compromise is achieved when the axial length of the or each magnets, the axial length of each polepiece and the radial xe2x80x9cthicknessxe2x80x9d of the air gap are all substantially the same.
In one embodiment of the present invention, the stator of the linear electromagnetic machine surrounds the armature, and has conductor coils housed within circumferential slots in a ferromagnetic stator body. The hoop resistance of the ferromagnetic stator body, that is the circumferential resistance with respect to induced electrical currents, is increased by means of at least one axially extending channel the depth of which is less than the thickness of the material of the stator body. Preferably the circumferential slots have a symmetrical cyclic contour in the axial direction so as to maintain a constant radial reluctance. The armature is preferably close-fitting to the stator and provided with suitable bearings.
In another aspect the present invention provides a linear electromagnetic machine according to any preceding claims, characterised in that the stator is located radially within the armature and the said ferromagnetic sleeve is located on the radially outer surface of the armature.
The axial spacing, electrical connections and impedances of the coils are so adapted as to permit the machine to be controlled by a suitable electronic drive unit as will be described in more detail below. The means for producing a radial magnetic field may comprise plane discs or rings of magnetic material permanently magnetised in a direction orthogonal to their flat surfaces. The rings are assembled in series along the central axis of the machine with like poles facing one another and having ferromagnetic polepieces interposed so as to concentrate the flux from the opposing magnets and to conduct that flux in a desired radial direction towards the electrical coils. A sleeve of radially-magnetised magnetic material may be positioned at the opposite radial periphery of the magnet rings and polepieces so as to oppose the conduction of the flux in the unwanted direction, there being a ferromagnetic sleeve fitted to said radially-magnetised material on the side thereof remote from the said rings and polepieces.
In another aspect the present invention provides a linear electromagnetic machine according to any preceding claim, characterised in that the coils follow circilinear or rectilinear axially sinuous path circumferentially around the said axis.
Thus, if the electromagnetic actuator includes or is associated with a position transducer providing a signal representative of the relative positions of the armature and the stator, that the electrical coils of the stator may be connected to an electronic drive unit for controlling the magnitude and direction of the currents in the coils so as to cause the desired axially-directed force to be created between the armature and the stator.
Preferably the armature and the stator are of circular cross section and are fitted with suitable gas seals that will permit the device to be pressurised so as to form part of a gas spring when required.
Preferably the electronic drive unit is arranged to produce a signal representative of the current supplied to the actuator, the integral of such signal being used to make adjustments to the pressure of the said gas spring in certain applications.
The most convenient configuration of a linear electromagnetic machine is to form the stator as a cylinder with the coils wound circumferentially and to form the armature with a cylindrical outer surface to slide axially within the inner cylindrical surface of the stator. This configuration may then provide a constant reluctance cylindrical stator.
It is a common error to consider that the piston moves because there is some sort of attractive or repulsive magnetic effect produced by the coils, acting on the armature, as there would be for a solenoid or electromagnet. This is not the case. A force exists in each coil which is the vector product of the current and the magnetic flux densityxe2x80x94a reaction force is therefore produced between them which is equal to the vector sum of all the coil forces.
Because the circular electric currents are arranged to be orthogonal to the radial magnetic flux, the forces produced are axial and not rotational. The machine may be considered as being electrically equivalent to a two pole rotary brushless AC servomotor and may therefore be controlled by a very similar drive system.
The principal difficulty in connecting a linear electromagnetic machine as defined herein to such a controller is that the cycle of phases repeats a few times only, within the limits of the linear displacement. This should be contrasted with the equivalent rotary machine, for which the cycle can be made to repeat indefinitely, corresponding with the rotation of the motor shaft. This difficulty is associated with the output signal from the transducer, which must be a linear device which does not always have a rotary equivalent. The linear position signal has to be converted to a cyclic phase command with the appropriate offset to produce optimum thrust.
The present invention also comprehends a linear electromagnetic machine adapted for use with a three phase supply, comprising a stator having a plurality of coils aligned along a common axis, an armature having means for producing a magnetic field extending radially of the said axis and control means for applying electrical currents to the coils the magnitude and direction of which result in the generation of a force between the armature and the stator substantially parallel to the said axis, characterised in that the magnetic field produced by the armature comprises at least two regions of opposite polarity defining the magnetic field period and the axial dimensions of each coil is substantially one sixth of the magnetic period of the armature.
The features of an embodiment of the present invention can be summarised thus:
The armature consists of an axially-alternating sequence of permanent magnets which produce a radial magnetic flux of polarity which alternates axially.
The axial distance between like pole centres on the armature is referred-to as the magnetic period of the machine.
The stator coils are stacked as a series of identical units, each of which has an axial dimension equal to one sixth of the magnetic period.
The coils of the stator are wired as three electrically-isolated phases.
The phases are constructed by allocating every third coil in axial sequence to the same phase and connecting all coils of the same phase in series, but alternately opposite in direction of current flow.
The second coil in each set of three adjacent coils is connected in the opposite sense to that of its neighbours.
The three phases are joined together at a star point, at the end of the distant from the input terminals.
The function of the output stage from the control device is to produce, for any position of the armature, three quasi-dc drive currentsxe2x80x94not three continuously-alternating currents
The drive currents are arranged to have a zero sum at all times, according to the common relationship equivalent to that of three sinusoidal signals differing in phase by 120xc2x0 from one another
The absolute phase reference of the drive currents is variable under external control
The position of the armature is not locked to the absolute phase of the coil currents. That is to say, the position of the piston will not follow the phase command signal at a constant drive current amplitude
There is an optimum absolute phasing of the coil currents for maximum thrust at any position of the armature relative to the coil stack. This optimum phasing is a linear function of the piston position and it repeats for every transit of the armature through a magnetic period
Changing the absolute phase of the coil currents by 180xc2x0 reverses the direction of the thrust produced by the armature
The actual position of the piston determines the current drive phasing, not the other way round.
The values of the drive currents at any time are defined by:
I(1)=A*sin(x+offset)
I(2)=A*sin(x+offset+2xcfx80/3)
I(3)=A*sin(x+offset+4xcfx80/3)
where A is the current drive signal,
x is the absolute (cyclic) phase command corresponding to the displacement of the piston from a datum position and offset is a preset phasing parameter that relates to the datum itself.
The value of the drive current demand signal is a function of the error between the commanded and actual value of the armature position, the time integral of that error, and the time derivative of that error, as calculated by the servo-controller module.
The ram coils are designed for a specific busbar voltage. That is to say, the coil impedances are determined by the current required to produce the design peak thrust, taking into account the highest speed required (i.e. the back emf value). Rams that are required to operate at high speeds will generally be capable of operating at high peak currents under static conditions and will require a power module that has a larger power rating. (A high speed ram has a greater power output than a slow ram.)
It should be noted that the function of the ram is usually to produce a thrust, rather than to maintain a preset position or a preset speed of movement (although these latter may be acquired in special circumstances. The machine can also be allowed to xe2x80x9cfreewheelxe2x80x9d when necessary, that is to continue its motion without any accelerating or braking thrust. This facility is important because it saves large amounts of energy when positioning inertial loads.
It will be appreciated that conventional PWM control applies a constant-frequency waveform of variable mark:space ratio to the load, thus appearing to be a low-impedance source of variable voltage. But if, because the permanent-magnet armature is moving, a back-emf is generated by the load coils, the current output will either be lessened (when the armature is moving in the direction of the force) or increased (when the directions of force and travel are opposed.) The inertia that is directly coupled to the armature of a ram can be very large and the power which can be delivered into a coil driver module under braking conditions is considerable.
The drive circuits must therefore be high-impedance current drivers, which can be arranged in two ways. In the first arrangement the pulse width modulator is of the conventional type but includes in the fast control loop a current-sensing element. By this means the voltage output of the modulator follows the back emf of the coils and does not appear as a driving or braking element when the current drive command is zero.
In the second arrangement the two parts of the bridge are pulsed separately. That is to say, for positive current drive only the upper half of the bridge is pulsed whilst the lower remains switched off, and vice-versa. For zero current both parts of the bridge are off and there are no spurious currents when the armature is moving. This system is inherently a high-impedance current drive. It should also be arranged to include a current-sensing element in the control loop, although the current sensor does not need to be as fast as for the responsive voltage-drive system.
It should be noted that, in contrast to fluid power rams, a linear electromagnetic machine formed as a ram is capable of moving an inertial load ballistically, only requiring power for acceleration and deceleration. In certain circumstances much of the deceleration energy may even be delivered back to the DC busbar. The ram efficiency in moving inertial loads may be still further increased by use of gas spring control described hereinafter briefly, and more fully described in the Applicant""s copending application PCT/GB98/02823.
If the linear electromagnet machine is connected to another machine, the machine element which is positioned by the armature may be fitted with a transducer of any suitable kind. There are, nevertheless, advantages in arranging for the ram to have its own transducer, fitted within the ram or mounted alongside.
One of the principal advantages of the ram is that it is capable of a very high positioning accuracy, since it has no backlash and no control transport lag. Whilst the overall stroke of the machine may be in the order of a meter, the positioning may be in the order of a micron; a resolution of 20 bits. This is likely to be in the noise of even the best analogue device, so that for any precision application a digital output signal is required from the transducer.
For less precise mechanisms, such as motion bases for entertainment or training purposes, the positioning accuracy needs only to be in the order of millimeters (perhaps 12 bits) and the performance of an analogue transducer is probably adequate. Linear potentiometers are normally designed to work immersed in oil, whereas the electromagnetic ram is dry, so a different, non-contacting type of transducer is to be preferred.
It will be understood that the coil assembly of the stator has to be mounted within the outer cylinder, so that it must either be loaded into the cylinder from one end or the cylinder must be split and the two halves then clamped onto the outside of the coil assembly. In either case it will be appreciated that it is very difficult to ensure firm metal-to-metal contact everywhere between the inner surface of the cylinder and the outer surfaces of the steel from which the slotted stator is built.
It is also necessary to convey large axial forces from the coils into the outer steel cylinder with crushing forces on the surrounding coils. The whole coil assembly is often cast in araldite resin to achieve this but a fault in any one coil then causes the complete assembly to be scrapped.
It is desirable to provide a firm metal-to-metal contact between the radially-directed ferromagnetic elements and the axially-directed ferromagnetic elements of the flux path in the stator and to allow each coil module to be separately manufactured, inspected and serviced if necessary.
In this embodiment the stator of an electromagnetic ram having a slotted steel coil assembly is constructed from a stack of modules having a cylindrical symmetry in that each module comprises a ferromagnetic washer or ring which abuts in the axial direction a coil of wire having an internal diameter approximately equal to the internal diameter of the washer, there being also abutting to the washer in the axial direction a ferromagnetic spacer ring surrounding the coil whose outer diameter is equal to that of the outer diameter of the ferromagnetic washer and whose axial dimension is accurately determined and is approximately equal to the axial dimension of the coil.
Preferably the stack of coil modules forming the stator is clamped together in the axial direction by tie rods outside the ram between end pieces of the ram stator assembly.
Preferably the washers are shaped to a single or repeated ramp form so as to present a constant reluctance to a radially-directed armature magnetic circuit.
Preferably the washers are slotted in a radial direction so as to allow the innermost coil wire to be brought to the outer surface of the stator assembly.
Preferably the spacer ring is notched to allow the end of the outermost coil layer to be brought to the surface of the stator assembly.
Preferably the rings and washers have a detent or alignment device on their outer surfaces so as to facilitate the correct assembly of a stack of coil modules.
Referring now to FIG. 9, which shows a stack of six coil modules, the coils 1 are shown sandwiched between washers 52 and surrounded on their outer surfaces by spacer rings 53. The modules are arranged to present a constant reluctance to a radially-directed magnetic field from a permanent-magnet armature moving within the washers 2 and for this reason the modules are skewed by one coil axial length as shown. It will be understood that to facilitate the manufacture of such modules the washers may be assembled from two or more layers of stamped material and that the coils and spacers may also be subdivided in an axial direction if required.